1. Field of the Invention
The present invention relates to any physical process that is carried out in a closed system (sometimes referred to as a “black box”) with input and output streams. The primary purpose of this invention is to characterize and optimize specific responses or output streams from complex industrial processes. The invention is demonstrated here in two case studies that characterize NOx emissions from different coal-fired boilers for the energy industry; however, it is applicable to characterize any measurable conserved quantity (e.g., mass, energy and momentum) in all industrial processes. This invention permits the study of complex commercial processes without changing the normal configuration, using short-duration tests, and with high accuracy. Accordingly, this invention may be applied to define best operating practices for all industrial processes or, through automation, be applied to give intelligent feedback control.
2. Description of the Related Art
The current industrial testing methods employed to characterize or optimize complex processes include single parameter tests, factorial-design tests, fractional factorial screening tests, mixture-design tests, simplex optimization methods, and neural network optimization methods. All of these experimental methods require long duration tests to obtain statistically significant results that are process independent. These methods are used sparingly because of the difficulty and high cost associated with completing such tests in commercial-scale process equipment.
The particular application reported here is for coal-burning furnaces used for power and industrial steam generation. This is predominantly important in electric utility power generation applications, which are highly regulated by Federal and State environmental authorities. The method of the present invention falls within the field of optimization of the staged combustion process for reducing NOx emissions and maintaining or improving furnace performance. Optimization requires feedback for control of the diverse equipment in the fuel delivery system designs used by the industry.
Fuel and Air Delivery System Equipment Control
Coal-fired boilers typically have multiple arrangements of coal pulverizing mills, each mill supplying coal through multiple pipes to multiple burners within the boiler. Each parallel coal supply path typically originates at a respective pulverizer mill and terminates at the individual burner mounted in the boiler. Each coal pipe has its own characteristic mechanical system performance/resistance properties for the two phase flow of primary air and coal, and this varies for each parallel coal pipe path at any given time and boiler load, based upon a number of system factors relating to both equipment and process variables. For example, equipment such as a forced draft fan, air heater, mill exhauster fan, coal feeder, coal pulverizer, coal classifier, riffle box/splitter, fixed orifice, piping system, air flow and coal flow monitor, coal damper, burner isolation valve, burner, boiler, and process parameters such as elevation, air temperature, air pressure, air flow, coal flow, coal density, coal moisture, coal composition and coal particle size all impact the performance/resistance characteristics of the fuel delivery system. In other words, as the boiler load changes and as the individual mechanical factors vary for each coal pipe, the resistance changes for the total coal delivery system and each individual coal pipe within that system.
Coal flow balancing of multiple burners is a difficult problem for plant engineers and operators. It is typically performed as an iterative series of manual coal flow measurements and adjustments of flow restrictive devices in the coal piping. With the introduction of manual coal dampers, coal flow has been balanced by adjusting each manual damper in each of the pulverized coal pipes that supplies the burners from a single mill. The coal flow rates in each pipe are measured manually by sampling with a coal probe traversing across the coal pipe area. While this approach has the potential to achieve approximate balance, changes in fuel consumption, operating conditions and wear on the orifice plates result in uncontrollable coal flow balance variations. However, as a problem attendant to the use of coal flow and air flow control, oxygen is increased in the coal mixture, exacerbating the NOx production problem.
Combustion System Designs for NOx Emission Control
Two-stage combustion methods are combustion techniques (NOx reduction techniques) for reducing the concentration of NOx generated in the furnace exhaust gas. The two-stage combustion methods are classified into the following two approaches. One approach is to reduce the NOx generation of a furnace as a whole, while the other approach is to reduce the NOx generation of a single burner.
In the approach to reduce the NOx concentration of a furnace as a whole, the air ratio (ratio of the amount of supplied air to the amount of necessary air for completely combusting an amount of fuel; the air ratio of unity corresponds to one stoichiometric equivalent) in the burner zone of the furnace is maintained below unity. In this fuel-rich condition, generated NOx is chemically reduced, and hence NOx reduction is achieved. The unburned carbon resulting from this approach is completely combusted with air added through an air inlet provided downstream of the burner zone.
In the approach to reduce the NOx generation of a single solid fuel burner (simply a burner, in some cases hereafter) such as a pulverized-coal burner, secondary and tertiary air flows are swirled, thereby delaying the mixing thereof with the flow of pulverized-coal burning with primary air alone. By virtue of this, a large chemical reduction region is formed (such a burner is called a low-NOx burner, hereafter).
These techniques have achieved a reduction of NOx concentration in the exhaust gas down to 130 ppm (fuel ratio=fixed carbon/volatile matter=2, nitrogen content in the coal=1.5%, and unburned carbon content in the ash=5% or less). Nevertheless, the regulated value of NOx concentration in the exhaust gas has been tightened year by year, and the required value of NOx concentration in the exhaust gas for the near future is 100 ppm or less.
Low NOx burners capable of reducing NOx generation down to 100 ppm or less have been developed. Such burners include: a burner having an internal flame stabilizing zone for reinforcing the NOx-reduced combustion in the burner section; and a burner having a flame stabilizing region for bridging between an internal flame stabilizing zone as described above and an external flame stabilizing zone provided in the outer periphery of the combustion nozzle through which the mixture of pulverized coal and carrier gas flows.
These designs are inherently reserved for new furnaces. However, in existing designs in the United States, retrofitting is a more economical objective for operating systems. In addition, the achieved ratios of air to coal in these units does not represent the optimal, achievable rates.